The air-fuel ratio sensor is a sensor that is provided for a detection of the exhaust gas of the internal-combustion engine, and for a control of the air-fuel ratio of a fuel-air mixture that is supplied to the internal-combustion engine to a desired value, which outputs a detection signal that changes according to the density of the exhaust gas.
Generally, when the detected density of the exhaust gas is a stoichiometric value (i.e., a theoretical air-fuel ratio), the electric current in the sensor is equal to 0 [A], with a bi-directional flow of the electric current flowing therein according to a rich and lean state of the combustion.
For example, a current sensing resistor is used for detection of (i.e., to sense) a sensor current that flows in the air-fuel ratio sensor, and a fuel injection control is appropriately performed based on a detected voltage between the terminals of the current sensing resistor in which the sensor current flows.
An example of a controller of the air-fuel ratio sensor is shown in a patent document, JP H11-230931 A (patent document 1) as an air-fuel ratio sensing device. In the art of the patent document 1, an electric current is provided from each of the two operational amplifiers, i.e., respectively from the output terminals, to each of the terminals of the air-fuel ratio sensor via the resistors, for an application of a voltage to each of the two terminals of the air-fuel ratio sensor.
According to the method of the patent document 1, for a feedback of the voltage of the sensor element, a feedback resistor is provided at a position between the output terminal of a drive circuit and the inverted input terminal of the operational amplifier, and an electric current flowing in the sensor element is detected by the current sensing resistor, and the voltage and the electric current output from the two operational amplifiers are controlled based on the detected voltage.
However, in such a configuration, it takes time for the application voltage applied to the sensor element to have a stable voltage value, which is not preferable. Further, even when a capacitor element is connected to the above-mentioned feedback resistor for a phase compensation, it still takes time to control. i.e., to adjust, the application voltage to a target voltage value.